Structural and Electronic Evolution in Metal-Insulating Nickelates Probed by Advanced Cryo-STEM Techniques

Correlated phase transitions offer both a rich playground and unique challenge for real-space imaging and spectroscopic studies down to the atomic scale. With direct access to relevant order parameters such as atomic lattice, charge, and spin, scanning transmission electron microscopy (STEM) can provide crucial information regarding phase coexistence, evolution, and inhomogeneity compared to bulk-averaged probes. Recent technical and analytical improvements are now enabling correlated structural and electronic studies down to liquid nitrogen temperatures in the STEM. More stable and flexible cryogenic sample holders, for example, slow thermally driven mechanical drift at the sample and provide additional tuning knobs such as variable temperature or biasing control [1]. Fast, low-noise detectors equipped on instruments with high-brightness sources improve the quality of dose-limited signals [2,3,4]. Analytically, new methods of dimensionality reduction can help tease out electronic phases from low signal-to-noise-ratio data [5]. Here, we study the competition and interplay between structural and electronic effects across the metal-insulator transition (MIT) in free-standing rare-earth nickelate perovskites (RNiO₃) membranes through a combination of cryogenic four-dimensional (4D)-STEM, multislice electron ptychography (MEP), and electron energy loss spectroscopy (EELS). We observe clear spectroscopic signatures consistent with the temperature-driven MIT which are resolved into spatially separated components templated by the film (membrane) geometry, while imaging and local diffraction reveal the importance of local domain orientation. Mapping the length scales and coupling of such structural and electronic transitions will have important implications for future applications of these materials in micro- and nanoscale devices for e.g. neuromorphic computing.<br/><br/>1 Goodge, et al. <i>Microsc. & Microan.</i> 26 (3), 439-446 (2020).<br/>2 Goodge, et al. <i>arXiv</i>: 2007.09747 (2020).<br/>3 Philipp, et al. <i>Microsc. and Microan.</i> 28, (2), 425–440, (2022).<br/>4 Goodge, et al. <i>Microsc. & Microan. </i>27 (S1), 2704-2705 (2021).<br/>5 Colletta, et al. <i>Microsc. & Microan.</i> <b>29</b> S1, 394–396 (2023).